Flange bolts are used to press together various components of a turbine, as for example a turbine casing, pipes or other joints.
A turbine casing comprises flanges that are integral parts of the casing and intended to hold together the upper and lower halves of the casing when it is under pressure. One half of the casing comprises an upper flange half and the other half a lower flange half, which are pressed together by flange bolts and flange nuts. The two flange halves are pressed together by means of bolting forces, which ensures that no steam or gas leaks out of the casing. A cross-section of a casing of a gas or steam turbine with such flange bolts is shown in FIG. 1. The turbine is enclosed by an inner casing having an upper half 31 and a lower half 33, which are held together by a flange having an upper and lower half 32 and 34, respectively. The two flange halves are an integral part of the inner casing and are pressed together by the bolting force given by the bolts and nuts 35 so that no fluid leaks out at the flange plane 36 between the upper and lower half of the inner casing. Similarly, the outer casing comprises an upper half 21 and a lower half 23, which are pressed together by a flange with an upper half 22 and lower half 24. The upper and lower flange halves 22 and 24 are pressed together by bolts and nuts 25 such that no steam or gas leaks out at the flange plane 26 that extends between the upper and lower halves of the outer casings.
At elevated temperatures the bolts can relax and the bolting forces diminish as a function of time. If the temperature is too high, the bolting forces diminish at such a fast rate that the bolts no longer keep the outer casing closed sufficiently tight.
U.S. Pat. No. 6,273,675 discloses an arrangement for cooling a flange applied to the outer casing of a steam turbine comprising pipes placed in the insulation of the casing and contacting the outer edge of the flange. Heat is transferred by natural convection to the air in the pipes. The bolts are cooled indirectly via the flange and the pipe in the insulation. The same patent discloses another known method of cooling the outer casings, where the flange contains a boring, through which air can flow and cool the flange by natural convection.
These cooling arrangements are arranged in the flange such that primarily the flange is cooled and the bolts are only cooled indirectly via the flange. The steam inside the casing warms the flange, which limits the cooling effect on the bolts via the flange. In addition, the cooling effect by means of natural convective airflow is limited due to its limited flow velocity. These arrangements are furthermore associated with a relatively large heat loss because much heat has to be transported out of the flange in order to cool the bolts. These cooling arrangements are only applicable to the outer casings of turbines. They are not suitable for inner casings because, typically steam flows around the inner casings, whereby cooling air is not available for convective cooling.
It is furthermore known in turbine technology to provide bolts with a central boring in order to allow heating of the bolts using heating rods that are inserted into the central boring. The method furthermore allows measurement of the bolt's length before and after the installation of the turbine. The bolts are heated and thereby lengthened in order to allow the turning of the nuts by a given predetermined distance without force. Prior to the start of turbine operation, the heating rods are removed and the bolts cool off. The bolts then have an initial tension. The heating of the bolts by this method enables a sufficient tightening of the bolts.